1. Field of the Invention
The present invention relates to an autofocus device.
2. Description of the Related Art
Heretofore, an autofocus device, which is provided in an image measurement machine or an optical instrument, has been known.
The autofocus device condenses a single light beam onto a measurement target (work), performs focus detection, and adjusts a focusing position.
Specifically, for example, in an autofocus device 500 shown in FIG. 8, a laser beam light emitted from a laser diode 501 (emitted light) is irradiated onto a work W through a half mirror 502, a tube lens 511 and an objective lens 513. Object light, which is the emitted light reflected on a surface of the work W and returned through the objective lens 513 and the tube lens 511, is split into two by a beam splitter 504, and two pieces of light are made incident onto photo acceptance units 505A and 505B. At this time, on the surface of the work W, an image of a micro spot S is formed.
In the autofocus device as described above, for example, as shown in FIG. 9A and FIG. 9B, if the surface of the work W has a level difference, when the focus detection is performed at the boundary of the level difference (edge line E), a single light beam irradiated onto the edge line E is scattered by the edge line E. Accordingly, there is a problem that the focus detection cannot be performed.
Therefore, for example, as shown in FIGS. 10A and 10B, there is proposed a technology for forming a line-like image L on the surface of the work W by using a cylindrical lens 512 (for example, refer to Japanese Patent Laid-Open Publication No. 2006-276320 as Patent Literature 1). In this case, even if a part of the beam is scattered on the edge line E, the other part of the beam reflected on the other part of the line-like image L can be detected, and focus detection by using the average distance to the surface of the work W can be performed (which is hereinafter referred to as “averaging effect”).
However, in the invention described in Patent Literature 1 described above, for example, as shown in FIG. 10C and FIG. 10D, the entire beam is scattered depending on an orientation (direction of the level difference) of the work W in such a case where a direction of the edge line E of the level difference and a direction of the line-like image L coincide with each other. Therefore, in some cases, it is difficult to perform the focus detection.